Problem: $ { {-5} \times \left[\begin{array}{rr}{1} & {-2} \\ {4} & {1}\end{array}\right] = {?} } $
To multiply a matrix by a number, multiply each item of the matrix by the number. ${\left[\begin{array}{rr}{-5}\times{1} & {-5}\times{-2} \\ {-5}\times{4} & {-5}\times{1}\end{array}\right]}$ Now, simplify: ${\left[\begin{array}{rr}{-5} & {10} \\ {-20} & {-5}\end{array}\right]}$